Semiconductor memory device

ABSTRACT

According to embodiments, a semiconductor memory device includes a first electrode, a second electrode, a memory cell, and a control circuit. The memory cell is provided between the first electrode and the second electrode and includes a metal film and a resistance change film. The control circuit applies a voltage between the first electrode and the second electrode to perform transition of a resistive state of the memory cell. The control circuit performs a first writing operation by applying a first pulse having a voltage of a first polarity to the memory cell and applying a second pulse having a voltage of the first polarity smaller than the voltage of the first pulse to the memory cell continuously after applying the first pulse.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit and priority under 35 U.S.C. § 120 toU.S. patent application Ser. No. 15/269,506 filed on Sep. 19, 2016entitled “Semiconductor Memory Device”, which claims the benefit of U.S.Provisional Patent Application No. 62/309,676, filed on Mar. 17, 2016,the entire contents of which are incorporated herein by reference.

BACKGROUND Field

Embodiments described herein relate generally to a semiconductor memorydevice.

Description of the Related Art

As a semiconductor memory device that stores large capacity data foruses, attention has been focused on such as a resistive random accessmemory (ReRAM: Resistive RAM) that is easily formed three-dimensionally.As such resistive random access memory, CBRAM (Conduction Bridge RAM),for example, is used.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a semiconductor memory device according toa first embodiment.

FIG. 2 is a circuit diagram illustrating a configuration of a memorycell array of the semiconductor memory device.

FIG. 3 is a perspective view illustrating the configuration of thememory cell array.

FIG. 4 is a cross-sectional view illustrating a configuration of amemory cell of the semiconductor memory device.

FIG. 5 is a schematic diagram for describing a set operation of thesemiconductor memory device.

FIG. 6 is a schematic diagram for describing a reset operation of thesemiconductor memory device.

FIG. 7 is a circuit diagram for describing the set operation of thesemiconductor memory device.

FIG. 8 is a timing diagram for describing the set operation.

FIG. 9 is a timing diagram for describing a set operation according to acomparative example.

FIG. 10 is a timing diagram for describing a set operation according toa modification of a first embodiment.

FIG. 11 is a circuit diagram for describing a set operation according toa second embodiment.

FIG. 12 is a timing diagram for describing the set operation.

FIG. 13 is a flowchart for describing a set operation according to athird embodiment.

FIG. 14 is a timing diagram for describing the set operation.

FIG. 15 is a flowchart for describing a set operation according to amodification of a third embodiment.

FIG. 16 is a timing diagram for describing the set operation.

FIG. 17 is a flowchart for describing a set operation according to amodification of the third embodiment.

FIG. 18 is a timing diagram for describing the set operation.

FIG. 19 is a timing diagram for describing another example of the setoperation.

FIG. 20 is a timing diagram for describing a set operation according toa fourth embodiment.

DETAILED DESCRIPTION

A semiconductor memory device according to one embodiment includes afirst electrode, a second electrode, a memory cell, and a controlcircuit. The memory cell is provided between the first electrode and thesecond electrode, and includes a metal film and a resistance changefilm. The control circuit applies a voltage between the first electrodeand the second electrode to perform transition of a resistive state ofthe memory cell. The control circuit performs a first writing operationby applying a first pulse to the memory cell and applying a second pulseto the memory cell continuously after applying the first pulse. Thefirst pulse has a voltage of a first polarity. The second pulse has avoltage of the first polarity smaller than the voltage of the firstpulse.

Next, a description will be given of the semiconductor memory deviceaccording to the embodiments in detail with reference to the drawings.Here, these embodiments are only examples, and are not described for thepurpose of limiting the present invention.

For example, the drawings used in the following embodiments areschematically illustrated. The thickness, the width, and the ratio ofthe layer, and the magnitude, the application time, and the ratio of thevoltage, or similar parameter may be different from actual parameters.

In the following description, as a structure of a memory cell array, aconfiguration including one layer of memory mat is indicated as anexample. However, the present invention may be applicable to aconfiguration including two or more laminated layers of memory mats. Thepresent invention may also be applicable to a configuration with a bitline extending to a direction intersecting with a surface of asubstrate, which is so-called VBL (Vertical Bit Line) type.

In the following description, when a certain voltage is larger (smaller)than another voltage, it means that an amplitude of the certain voltageis larger (smaller) than an amplitude of the other voltage. And when acertain voltage is higher (lower) than another voltage, it means thatthe certain voltage is higher voltage (lower voltage) than the othervoltage. For example, the voltage with −1 V is smaller than the voltagewith −2 V, and higher than the voltage with −2 V.

First Embodiment

[Overall Configuration]

FIG. 1 is a block diagram of a semiconductor memory device according toa first embodiment. Here, the semiconductor memory device means, forexample, a chip C that stores user data, an electronic device thatincludes the chip C (such as a smart phone, a mobile phone, a tabletterminal, a music player, a wearable device), or the like. The user datais data to express, for example, contents used by a user (such as acharacter string, an audio, an image and a moving image), which areexpressed by combinations of “0” and “1.”

The chip C includes a memory cell array 11, a row decoder 12, a columndecoder 13, an upper block decoder 14, a power supply 15, and a controlcircuit 16.

The memory cell array 11 includes a plurality of memory cells MC thatstore data of 1 bit or a plurality of bits.

The memory cell array 11 is configured such that an access (dataerasure/writing/reading) to a desired memory cell MC can be performed byapplying a predetermined voltage to desired bit line BL and word line WLcorresponding to address data.

The row decoder 12 selects one word line WL corresponding to a rowaddress for access to apply the predetermined voltage.

The column decoder 13 selects one or a plurality of bit lines BLcorresponding to a column address for access to apply a predeterminedvoltage. The column decoder 13 includes a sense amplifier circuit fordetecting the current or the voltage of the bit line BL to determine theuser data recorded on the memory cell MC.

The upper block decoder 14 generates the row address and the columnaddress corresponding to the address data to supply to the row decoder12 and the column decoder 13.

The power supply 15 generates combinations of the predetermined voltagescorresponding to each operation of data erasure/writing/reading tosupply to the row decoder 12 and the column decoder 13.

The control circuit 16 accepts external control signals from an externalhost (not illustrated) to control such as the upper block decoder 14 andthe power supply 15.

FIG. 2 is an equivalent circuit diagram illustrating a configuration ofa part of the memory cell array 11.

The memory cell array 11 includes a plurality of bit lines BL (secondwiring), a plurality of word lines WL (first wiring), and a plurality ofmemory cells MC connected to these bit lines BL and word lines WL. Thesememory cells MC are connected to the column decoder 13 (FIG. 1) via thebit line BL and to the row decoder 12 (FIG. 1) via the word line WL. Theplurality of memory cells MC connected to the common word line WL storeuser data of one page, for example. The plurality of memory cells MCeach store data of one bit, for example.

The memory cell MC serves as a variable resistive element to vary itsresistance value corresponding to the stored data. For example, thememory cell MC that records “0” is in a high resistance state, and thememory cell MC that records “1” is in a low resistance state.

FIG. 3 is a schematic perspective view illustrating a part of theconfiguration of the memory cell array 11. FIG. 3 is a drawing fordescribing a three-dimensional structure of the wiring, the memory cellsMC, and similar part. Then, the interlayer insulating films providedbetween such as the wirings are not illustrated. FIG. 3 is a drawing foran explanation. Then, the specific configuration may be changed asnecessary.

As illustrated in FIG. 3, the memory cell array 11 is, for example, whatis called a cross-point type memory cell array. That is, in the memorycell array 11, a plurality of bit lines BL arranged in an X direction,provided in parallel and extending in a Y direction is provided. Abovethe plurality of bit lines BL, a plurality of word lines WL arranged inthe Y direction, provided in parallel and extending in the X directionis provided. Furthermore, at the intersecting portions between theplurality of bit lines BL and the plurality of word lines WL, therespective memory cells MC are provided.

[Memory Cell MC]

FIG. 4 is a cross-sectional view illustrating a configuration of a partof the memory cell MC. In FIG. 4, interlayer insulating films providedbetween the likes of the wirings or the like are not illustrated.

The memory cell MC is provided between a lower electrode 101 (cathode,the first electrode) connected to the bit line BL and an upper electrode104 (anode, the second electrode) connected to the word line WL. Thememory cell MC includes a resistance change film 102 provided betweenthe lower electrode 101 and the upper electrode 104, and a metal film103 provided between this resistance change film 102 and the upperelectrode 104. In the memory cell MC, a conductive filament 105 isformed. The memory cell MC may be in two types of resistive states: alow resistance state and a high resistance state. In the low resistancestate, the filament 105 is formed within the resistance change film 102so as to penetrate the resistance change film 102. This filament 105becomes a current path. In the high resistance state, the filament 105is separated or disappears within the resistance change film 102, andthe current path is not constituted.

The lower electrode 101 is formed of a conductive film, for example,tungsten (W). The lower electrode 101 is provided on a top surface ofthe bit line BL and serves as a cathode of the memory cell MC. Theresistance change film 102 is formed of, for example, oxide such assilicon oxide (SiO₂) or alumina (Al₂O₃), or nitride such as siliconnitride (Si₃N₄). The metal film 103 is constituted of, for example,metal such as copper (Cu) or silver (Ag), or compound including one ormore types of metallic element. As an example, the metal film 103 isdescribed to be copper in the first embodiment. The metal film 103supplies metal ions, which constitute the filament 105, into theresistance change film 102.

The upper electrode 104 is formed of a conductive film, for example,tungsten (W). The upper electrode 104 is provided between the word lineWL and the resistance change film 102 and serves as an anode of thememory cell MC. The filament 105 is made of metal ions such as copperions (Cu⁺) or silver ions (Ag²⁺) supplied into the resistance changefilm 102 from the metal film 103.

FIG. 5 is a schematic diagram for describing a set operation thatperforms transition of the memory cell MC from the high resistance stateto the low resistance state. In the set operation, a voltage of apolarity in which the upper electrode 104 has a higher potential and thelower electrode 101 has a lower potential (hereinafter referred to as a“positive polarity” or a “first polarity”) is applied. This causes thecopper included in the metal film 103 to become positive ions to movetoward the lower electrode 101, which is a negative electrode. In thisrespect, the copper ions Cu⁺ diffuse to the lower electrode 101 throughvacancies V within the resistance change film 102. Then, the copper ionsCu⁺ combines with silicon atoms, oxygen atoms, or similar atoms withinthe resistance change film 102 to form the filament 105 within theresistance change film 102. This constitutes the current path betweenthe upper electrode 104 and the lower electrode 101, and the memory cellMC becomes in the low resistance state.

The vacancy V within the resistance change film 102 is a point defect,which occurs when a lattice of a material that constitutes theresistance change film 102 is destroyed, for example, due to anapplication of electric field to the resistance change film 102. Thecopper ion Cu⁺ has a large atomic size, therefore a barrier is high whenmoving between the lattices constituting the resistance change film 102.In view of this, the copper ion Cu⁺ needs electrically high energy tomove between the lattices. Here, if the vacancy V and a crystal defectare present at the proximity of the copper ion Cu⁺, the copper ion Cu⁺is allowed to move through the vacancy V, thus lowering the barrierbetween the lattices. In view of this, the diffusion speed of the copperion Cu⁺ depends on the vacancy concentration. That is, by increasing thevacancy concentration, the diffusion of the copper ions Cu⁺ with a lowelectrical energy supply is achieved.

FIG. 6 is a schematic diagram for describing a reset operation thatperforms transition of the memory cell MC from the low resistance stateto the high resistance state. In the reset operation, a voltage of aninverse polarity of the first polarity in which the lower electrode 101has a higher potential and the upper electrode 104 has a lower potential(hereinafter referred to as a “negative polarity” or a “secondpolarity”) is applied. This causes the copper included in the filament105 to become positive ions to move toward the upper electrode 104,which is a negative electrode. In this respect, the copper ions Cu⁺diffuse in a direction going away from the lower electrode 101 throughvacancies V within the resistance change film 102. This causes thefilament 105 of the resistance change film 102 to be separated ordisappear, and the current path disappears. Thus, the memory cell MCbecomes the high resistance state.

[Set Operation]

FIG. 7 is a circuit diagram for describing the set operation accordingto the embodiment. The following describes a case performing the setoperation for a selected memory cell MC_(S) connected to a selected bitline BL_(S) and a selected word line WL_(S).

FIG. 8 is a timing diagram for describing the set operation according tothe embodiment. FIG. 8 illustrates voltages applied to the selected wordline WL_(S) and the selected bit line BL_(S), a voltage and a current ofthe selected memory cell MC_(S), and voltages applied to a non-selectedword line WL_(U) and a non-selected bit line BL_(U).

At Timing T₁₀₁, a selected bit line voltage V_(BLS) (a fifth voltage) isapplied to the selected bit line BL_(S). The selected bit line voltageV_(BLS) is, for example, a voltage of the negative polarity, and has amagnitude to the extent where copper ions within a non-selected memorycell MC are almost unaffected. The selected bit line voltage V_(BLS) maynot be a voltage of the negative polarity, and, for example, may be setat around 0 V as illustrated in FIG. 10. As shown in FIG. 8, anintermediate potential between the selected bit line voltage V_(BLS) anda selected word line voltage V_(WLS2), which is described later, isapplied to, for example, the selected word line WL_(S), the non-selectedword line WL_(U), and the non-selected bit line BL_(U). This applies avoltage of the positive polarity |V_(BLS)| to the selected memory cellMC_(S).

At Timing T₁₀₂, a selected word line voltage V_(WLS1) (a first voltage)is applied to the selected word line WL_(S). The selected word linevoltage V_(WLS1) is a voltage of the positive polarity, and has, forexample, a magnitude to the extent where the copper ions within thenon-selected memory cell MC are almost unaffected. This applies a firstpulse P1 to the selected memory cell MC_(S).

The first pulse P1 has a voltage of the positive polarity V_(P1) with amagnitude to the extent where copper ions and oxygen ions within theselected memory cell MC_(S) are movable. Therefore, in the selectedmemory cell MC_(S), the vacancies V are pulled to the proximity of thelower electrode 101, thus increasing a concentration of the vacancies Vat the proximity of the lower electrode 101. The first pulse P1 has apulse width (an application time of the voltage) t_(P1) to the extentwhere the oxygen ions within the selected memory cell MC_(S) aremovable. However, a period between Timing T₁₀₂ and Timing T₁₀₃ is aperiod insufficient for moving the copper ions Cu⁺ to form the filament105. Thus, the selected memory cell MC_(S) does not transition to thelow resistance state.

At Timing T₁₀₃, the voltage of the selected word line WL_(S) isdecreased from the selected word line voltage V_(WLS1) (a first voltage)to a selected word line voltage V_(WLS2) (a second voltage). Theselected word line voltage V_(WLS2) is a voltage of the positivepolarity, and smaller than the selected word line voltage V_(WLS1). Thisapplies a second pulse P2 to the selected memory cell MC_(S). The secondpulse P2 has a voltage of the positive polarity V_(P2) with a magnitudeto the extent where the copper ions Cu⁺ within the selected memory cellMC_(S) are movable. Therefore, the copper ions Cu⁺ within the selectedmemory cell MC_(S) approach the lower electrode 101 through thevacancies V. The voltage V_(P2) of the second pulse P2 is smaller thanthe voltage V_(P1) of the first pulse P1. The second pulse P2 has apulse width (an application time of the voltage) t_(P2) to the extentwhere the copper ions Cu⁺ within the selected memory cell MC_(S) aremovable. The pulse width t_(P2) of the second pulse P2 is longer thanthe pulse width t_(P1) of the first pulse P1. In other words, the periodt_(P1) of applying the first pulse P1 to the selected memory cell MC_(S)is shorter than the period t_(P2) of applying the second pulse P2.

At Timing T₁₀₄, the selected memory cell MC_(S) becomes in the lowresistance state, and the current starts flowing through the selectedmemory cell MC_(S).

At Timing T₁₀₅, the voltage of the selected word line WL_(S) is set to 0V. In other words, applying the selected word line voltage V_(WLS2) isterminated. This decreases the voltage applied to the selected memorycell MC_(S) to |V_(BLS).

At Timing T₁₀₆, the voltage of the selected bit line BL_(S) is set to 0V. In other words, applying the selected bit line voltage V_(BLS) isterminated. This makes the voltage applied to the selected memory cellMC_(S) 0 V.

[Effect]

Applying a large voltage or applying the voltage for a long time to thememory cell MC applies large stress to the memory cell MC to acceleratethe deterioration of the memory cell MC. The magnitude of the stressapplied to the memory cell MC is indicated by, for example, a magnitudeof an integral value of the voltage by time E₁ (FIG. 8) or an integralvalue of the current by time E₂ (FIG. 8).

In order to decrease the stress of the memory cell MC, it is onlynecessary to decrease the voltage applied to the memory cell MC or toshorten the application time of the voltage. However, in order to movethe copper ions Cu⁺ within the memory cell MC, it is necessary to applya certain magnitude of voltage or to apply the voltage for a certainperiod.

Here, as described with reference to FIG. 5, the copper ions Cu⁺repeatedly diffuse through the vacancies V within the resistance changefilm 102. In this respect, the existence of the vacancies V or thecrystal defects at the proximity of the copper ions Cu⁺ allows thecopper ions Cu⁺ to move through the vacancies V, thus lowering thebarrier between the lattices. Diffusion of the copper ions Cu⁺ throughthe vacancies V requires existence of the vacancies V at adjacentlattice positions. In view of this, the diffusion speed of the copperion Cu⁺ depends on the vacancy concentration. Furthermore, the barrierbetween the lattices is low in diffusion through the vacancies V.Accordingly, a lower electrical energy supply diffuses Cu⁺ ions.Therefore, by increasing the concentration of the vacancies V at theproximity of the lower electrode 101, the voltage value and the pulsewidth to move the copper ions Cu⁺ can be decreased.

Then, as described with reference to FIG. 8, in the set operationaccording to the embodiment, the first pulse P1 and the second pulse P2are continuously applied to the selected memory cell MC_(S). The firstpulse P1 has the voltage V_(P1) to the extent where oxygen ions withinthe memory cell MC are movable. Accordingly, applying the first pulse P1breaks the lattices constituting the resistance change film 102. Thisgenerates the vacancies V due to, for example, the point defects. Thatis, many lattices constituting the resistance change film 102 are brokenby applying a high voltage to the resistance change film 102, and theconcentration of the vacancies V can be increased. Additionally, byapplying the first pulse P1 and the second pulse P2 continuously, it ispossible to move the copper ions Cu⁺ before the vacancies V diffuse fromthe proximity of the lower electrode 101.

By such kind of method, compared with, for example, as illustrated inFIG. 9, a case where applying a pulse P2′ having a certain voltageV_(P2)′ and a pulse width t_(P2)′ to the selected memory cell MC_(S) inthe set operation, by decreasing the voltage V_(P2) of the second pulseP2 or the shorten the pulse width t_(P2), the integral value of thevoltage by time E₁ can be decreased. According to the embodiment, thevoltage applied to the selected memory cell MC_(S) immediately after theselected memory cell MC_(S) transitions to the low resistance state issmall compare with, for example, that according to the method in FIG. 9.Therefore, this can decrease the integral value of the current by timeE₂. Therefore, this can decrease the stress of the memory cell MC.

The oxygen atoms within the resistance change film 102 are stronglybound together the silicon atoms by a covalent bond. Therefore, formingthe vacancies V by moving the oxygen atoms requires a high voltage. Inview of this, the voltage V_(P1) of the first pulse P1 is larger thanthe voltage V_(P2) of the second pulse P2. The voltage V_(P1) of thefirst pulse P1 may be set to, for example, 1.5 times or more of thevoltage V_(P2) of the second pulse P2.

In order to decrease the stress of memory cell MC, the pulse widtht_(P1) (FIG. 8) of the first pulse P1 is preferred to be fully short.The pulse width t_(P1) of the first pulse P1 may be set to, for example,quarter or less of the pulse width t_(P2) of the second pulse P2.

In an example illustrated in FIG. 8, the voltage of the negativepolarity is applied to the selected bit line BL_(S), and 0 V is appliedto the non-selected word line WL_(U) and the non-selected bit lineBL_(U). However, for example, as illustrated in FIG. 10, the voltagewith 0 V may be applied to the selected bit line BL_(S), and thevoltages of the positive polarity V_(WLU) and V_(BLU) may be applied tothe non-selected word line WL_(U) and the non-selected bit line BL_(U)respectively. In this case, a selected word line voltage V_(WLS1)′ (afirst voltage) may correspond to the voltage V_(P1) of the first pulseP1. A selected word line voltage V_(WLS2)′ (a second voltage) maycorrespond to the voltage V_(P2) of the second pulse P2.

Second Embodiment

FIG. 11 is a circuit diagram for describing a set operation according toa second embodiment. Like reference numerals designate corresponding oridentical elements to those of the first embodiment, and therefore suchelements will not be further elaborated here.

According to the first embodiment, one set operation performs transitionof one selected memory cell MC_(S) to the low resistance state. Incontrast, according to the second embodiment, one set operation performstransitions of a plurality of memory cells MC to the low resistancestate. The following describes a case performing sequentiallytransitions of the selected bit line BL_(S), and memory cells MC₁, MC₂,and MC₃ connected to word lines WL₁, WL₂, and WL₃ respectively to thelow resistance state.

FIG. 12 is a timing diagram for describing the set operation accordingto the embodiment. FIG. 12 illustrates voltages applied to the memorycells MC₁, MC₂, and MC₃.

At Timing T₂₀₁, the first pulse P1 is applied to the memory cell MC₁.

At Timing T₂₀₂, applying the first pulse P1 to the memory cell MC₁terminates, and applying the second pulse P2 starts.

At Timing T₂₀₃, the first pulse P1 is applied to the memory cell MC₂ ina state where the second pulse P2 is applied to the memory cell MC₁.

At Timing T₂₀₄, applying the first pulse P1 to the memory cell MC₂terminates, and applying the second pulse P2 starts.

At Timing T₂₀₅, the first pulse P1 is applied to the memory cell MC₃ ina state where the second pulse P2 is applied to the memory cell MC₁ andthe memory cell MC₂.

At Timing T₂₀₆, applying the first pulse P1 to the memory cell MC₃terminates, and applying the second pulse P2 starts.

According to the embodiment, by performing the set operations for theplurality of memory cells MC in parallel, it is possible to performwriting of the user data at high speed. Additionally, by shifting timingof applying the first pulse P1 having comparatively a large voltagevalue, it is possible to perform the set operation stably.

According to the embodiment, for example, applying the first pulse P1 tothe memory cell MC₂ starts after applying the first pulse P1 to thememory cell MC₁ has terminated. However, termination of applying thefirst pulse P1 to the memory cell MC₁ and start of applying the firstpulse P1 to the memory cell MC₂ may be simultaneous.

Third Embodiment

FIG. 13 is a flowchart for describing a set operation according to athird embodiment. FIG. 14 is a timing diagram for describing the setoperation. Like reference numerals designate corresponding or identicalelements to those of the first embodiment, and therefore such elementswill not be further elaborated here.

As described with reference to FIG. 8, according to the firstembodiment, one set operation applies the second pulse P2 to theselected memory cell MC_(S) only once. In contrast, according to thethird embodiment, as illustrated in FIG. 13 and FIG. 14, whileincreasing the voltage of the second pulse P2, the second pulse P2 isapplied to the selected memory cell MC_(S) several times. Every timeapplying the second pulse P2 terminates, a verify operation thatconfirms whether or not the selected memory cell MC_(S) has becomes in adesired state is performed. In the example illustrated in FIG. 8, thedesired state is the low resistance state.

At Step S101, in a mode similar to that of the first embodiment, thefirst pulse P1 is applied to the selected memory cell MC_(S).

At Step S102, the second pulse P2 is applied to the selected memory cellMC_(S).

At Step S103, the verify operation is performed. In the verifyoperation, as illustrated in FIG. 14, a verify pulse VP is applied tothe selected memory cell MC_(S) to confirm whether or not the selectedmemory cell MC_(S) has transitioned to the low resistance state. Theverify pulse VP has a voltage of the positive polarity having amagnitude to the extent where the copper ions within the memory cell MCare almost unaffected. The voltage of the verify pulse VP is smallerthan the voltage of the second pulse P2. As illustrated in FIG. 13, atStep S103, when the memory cell MC has not transitioned to the lowresistance state, Step S104 is performed. When the memory cell MC hastransitioned to the low resistance state, the set operation terminates.

At Step S104, the voltage of the second pulse P2 is increased. Afterperforming Step S104, Step S102 and Step S103 are repeatedly performed.Accordingly, as illustrated in FIG. 14, the voltage of the second pulseP2 is increased.

The embodiment, while performing the verify operation (Step S103),causes the voltage of the second pulse P2 to increase. When the memorycell MC has transitioned to the low resistance state, applying thevoltage terminates. As illustrated in FIG. 14, in such mode, theintegral value of the voltage by time E₁ is further decreased and thestress of the memory cell MC can be decreased.

Note that, according to the embodiment, the first pulse P1 is appliedonly once. However, for example, as illustrated in FIG. 15 and FIG. 16,Step S101, Step S102, and Step S103 all may be repeated after increasingthe voltage of the second pulse P2 at Step S104. Additionally, asillustrated in FIG. 17 and FIG. 18, while increasing the voltage of thesecond pulse P2, the voltage of the first pulse P1 may be decreased(Step S104′). The first pulse P1 having the thus decreased voltage valueis sometimes referred to as a fourth pulse P4. Additionally, asillustrated in FIG. 19, the voltage of the first pulse P1 may beincreased. The first pulse P1 having the thus amplified voltage value issometimes referred to as a third pulse P3.

Fourth Embodiment

FIG. 20 is a timing diagram for describing a set operation according toa fourth embodiment. Like reference numerals designate corresponding oridentical elements to those of the first embodiment, and therefore suchelements will not be further elaborated here.

According to the embodiment, the first pulse P1 is achieved by a surgepulse. That is, according to the embodiment, the voltage of the selectedword line WL_(S) transitionally reaches a selected word line voltageV_(WLS1) (a first voltage) between Timing T₁₀₂ and Timing T₁₀₃, andconverges to a selected word line voltage V_(WLS2) (a second voltage) atTiming T₁₀₃. Such set operation can be achieved by using an existingcontrol circuit.

According to the embodiment, a surge voltage (a transitional voltage) inthe set operation of the selected word line WL_(S) (the upper electrode104 of the selected memory cell MC_(S)) may be larger than a surgevoltage of another wiring, for example, the selected bit line BL_(S)(the lower electrode 101 of the selected memory cell MC_(S)). Forexample, as described with reference to FIG. 10, when the voltage with 0V is applied to the selected bit line BL_(S), and the voltages of thepositive polarity V_(WLU) and V_(BLU) are applied to the non-selectedword line WL_(U) and the non-selected bit line BL_(U) respectively, thesurge voltage in the set operation of the selected word line WL_(S) maybe larger than a surge voltage at another wiring, for example, thenon-selected word line WL_(U) or the non-selected bit line BL_(U).

[Others]

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel methods and systems describedherein may be embodied in a variety of other forms: furthermore, variousomissions, substitutions and changes in the form of the methods andsystems described herein may be made without departing from the spiritof the inventions. The accompanying claims and their equivalents areintended to cover such forms or modifications as would fall within thescope and spirit of the inventions.

What is claimed is:
 1. A semiconductor memory device comprising: a firstelectrode; a second electrode; a memory cell provided between the firstelectrode and the second electrode, the memory cell including aresistance change film; and a control circuit that applies a voltagebetween the first electrode and the second electrode to performtransition of a resistive state of the memory cell, wherein in a firstwriting, the control circuit: applies a first voltage of a firstpolarity to the first electrode; applies a second voltage of a secondpolarity opposite from the first polarity to the second electrode; andafter the first voltage is applied to the first electrode, continuouslyapplies a third voltage of the first polarity smaller than the firstvoltage to the first electrode while the second voltage is applied tothe second electrode.
 2. The semiconductor memory device according toclaim 1, wherein the first voltage is a transitional voltage, and avoltage applied to the first electrode decreases from the first voltageto the third voltage.
 3. The semiconductor memory device according toclaim 1, wherein in the first writing, a voltage applied to the firstelectrode transitionally decreases from the first voltage to the thirdvoltage.
 4. The semiconductor memory device according to claim 1,wherein a voltage applied to the first electrode converges from thefirst voltage to the third voltage.
 5. The semiconductor memory deviceaccording to claim 1, wherein a period in which the first voltage isapplied to the first electrode is shorter than a period in which thethird voltage is applied to the first electrode.
 6. The semiconductormemory device according to claim 1, wherein in the first writing, atransitional voltage applied to the first electrode is larger than atransitional voltage applied to the second electrode.
 7. Thesemiconductor memory device according to claim 1, wherein the secondpolarity is opposite from the first polarity, and in the first writing,the control circuit: changes a voltage of the first electrode from aground voltage to the first voltage; and changes a voltage of the secondelectrode from the ground voltage to the second voltage.
 8. Thesemiconductor memory device according to claim 1, wherein the firstvoltage, the second voltage and the third voltage are not less than aground voltage.
 9. The semiconductor memory device according to claim 1,wherein after the third voltage is applied to the first electrode, thecontrol circuit changes a voltage of the first electrode from the thirdvoltage to a ground voltage while the second voltage is applied to thesecond electrode.
 10. A semiconductor memory device comprising: a firstelectrode; a second electrode; a memory cell provided between the firstelectrode and the second electrode, the memory cell including aresistance change film; and a control circuit that applies a voltagebetween the first electrode and the second electrode to performtransition of a resistive state of the memory cell, wherein in a firstwriting, the control circuit applies a voltage of a first polarity tothe first electrode and a voltage of a second polarity opposite from thefirst polarity to the second electrode, and a first potential differencebetween the first electrode and the second electrode at a first timingis larger than a second potential difference between the first electrodeand the second electrode at a second timing after the first timing. 11.The semiconductor memory device according to claim 10, wherein apotential difference between the first electrode and the secondelectrode continuously decreases from the first potential difference tothe second potential difference.
 12. The semiconductor memory deviceaccording to claim 10, wherein a voltage applied to the first electrodeconverges from a voltage applied at the first timing to a voltageapplied at the second timing.
 13. The semiconductor memory deviceaccording to claim 10, wherein a constant voltage is applied to thesecond electrode.
 14. The semiconductor memory device according to claim10, wherein a constant voltage is applied to the second electrode in aperiod between the first timing and the second timing.
 15. Asemiconductor memory device comprising: a first electrode; a secondelectrode; a memory cell provided between the first electrode and thesecond electrode, the memory cell including a resistance change film;and a control circuit that applies a voltage between the first electrodeand the second electrode to perform transition of a resistive state ofthe memory cell, wherein in a first writing, the control circuit, whilea first voltage of a first polarity is applied to the second electrode,applies a second voltage of a second polarity opposite from the firstpolarity to the first electrode in a first period, and, continuouslyapplies a third voltage of the second polarity smaller than the secondvoltage to the first electrode in a second period longer than the firstperiod.
 16. The semiconductor memory device according to claim 15,wherein the control circuit, after the third voltage is applied to thefirst electrode, while a voltage of the first electrode is a groundvoltage, applies the first voltage to the second electrode in a thirdperiod.
 17. The semiconductor memory device according to claim 15,wherein a first voltage difference between the first voltage and thesecond voltage is larger than a second voltage difference between thefirst voltage and the third voltage.
 18. The semiconductor memory deviceaccording to claim 15, wherein the second voltage is a transitionalvoltage, and a voltage applied to the first electrode converges from thesecond voltage to the third voltage.